In recent years, with the progress of automated platemaking in lithographic printing, lithographic printing plates more excellent in flatness, as well as lithographic printing plates having better printing performance, have been required in order to make it possible to cut and accumulate them more stably and at higher speed, complying with automated platemaking and mass production.
The flatness is disclosed in JP-A-8-104069 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The above-mentioned patent proposes a material specified in tensile strength and warping in a rolling direction in a coil-like raw plate of an aluminum alloy plate after final cold rolling, and describes that the coil-like aluminum raw plate having flatness so as not to induce exposure deviation of a lithographic plate can be supplied thereby even in an automatic conveying step of a lithographic photosensitive printer.
In the technique disclosed in the above-mentioned patent, the printer using the support for the lithographic printing plate is stably automated by specifying the characteristics of the raw material. However, particularly in recent years, it is necessary to improve the flatness of the lithographic printing plates for increased efficiency of exposure and development, as well as further improvement in printing performance of the lithographic printing plates, and also in terms of cost, further improvement in productivity of the lithographic printing plates has been earnestly desired. Accordingly, even the technique disclosed in the above-mentioned patent is not said to be sufficient yet.
On the other hand, aluminum and aluminum alloys are used as aluminum supports for printing plates, particularly supports for lithographic printing plates.
In general, in order to use aluminum plates as substrates for lithographic printing plates, it is necessary that they have appropriate adhesion to photosensitive agents and water receptivity, and that surfaces thereof are uniformly roughened. The uniformly roughened surfaces require that the size of pits formed is appropriately uniform and such pits are uniformly formed on the whole surfaces. The pits significantly influence the scumming reduction and the printing durability, the printing performances of plate materials, and good or bad thereof is an important factor in the production of the plate materials.
JP-A-6-92052 proposes an invention comprising the steps of mechanically roughening a surface, followed by etching within the range of 0.5 to 30 g/m.sup.2, and performing pulse energizing of 200 to 600 c/dm.sup.2. Further, JP-A-7-9776 proposes to conduct etching in 1 to 5 g/m.sup.2 after mechanical surface roughening, and performing electrochemical surface roughening at an alternating current quantity of electricity of 300 to 800 c/dm.sup.2. JP-A-6-24166 proposes an invention comprising the steps of mechanically roughening a surface, followed by etching within the range of 0.5 to 30 g/m.sup.2, and conducting alternating current electrolysis at 200 to 600 c/dm.sup.2. Methods for roughening surfaces of substrates include mechanical surface roughening, chemical etching and electrochemical surface roughening. In JP-A-6-24166, an invention is also disclosed in which various conditions of mechanical surface roughening, chemical etching and electrochemical surface roughening are changed. That is, it proposes that a surface is chemically etched in 0.5 to 30 g/m.sup.2 after mechanical surface roughening, electrochemically roughened by giving an appropriate current density and quantity of electricity, then, etched within the range of 0.1 to 10 g/m.sup.2 to smooth edges, and subjected to anodization.
In addition, U.S. Pat. Nos. 4,427,500 and 4,581,996 both corresponding to JP-B-3-42196 (the term "JP-B" as used herein means an "examined Japanese patent publication") specify the shape of a roughened surface obtained by preliminarily graining of a base material to a center line average roughness of up to 0.1 .mu.m.
The above-mentioned inventions are excellent inventions. However, printing plates of higher quality have been desired from recent customers' needs, and the development of supports for lithographic printing plates fitting the needs have been desired. Further, it is necessary to decrease the production cost to a maximum.
In JP-A-6-92052 and JP-A-6-24166, no preliminary graining is conducted, so that the surface roughness after rolling is rough. When mechanical surface roughening, chemical etching and electrochemical surface roughening are applied to original aluminum having projecting streaks, photosensitive layers on projections become thin in sections of projecting streaks or in sections whose roughness is rough after coating, resulting in the development of disadvantages such as a reduction in printing durability and poor appearance at the sections. Further, JP-B-3-42196 discloses that the base material is preliminarily polished to a center line average roughness of up to 0.1 .mu.m. However, in order to carry out this, much labor and cost are required, and the production cost is sometimes increased very high.